The present invention relates to apparatuses and methods for determining a correspondence of a position with a reference position as can be used in particular for localizing or navigating mobile terminal devices in a wireless communication network.
For localizing mobile terminal devices, different locating technologies are available. The probably most well-known system for localization or navigation outdoors is the satellite-aided global positioning system (GPS). For localization or navigation within buildings or indoors, different approaches are known, such as, for example, infrared systems, RFID systems or also field strength evaluations of IEE 802.11 WLAN networks (WLAN=wireless local area network). Currently, the GPS system is only reliably available for outdoors. Latest extensions, such as highly sensitive receivers or the so-called A-GPS (assisted GPS) represent attempts to make the technology also useable for within buildings. A-GPS combines the usage of the satellite-based GPS system with a reception of so-called assistance information from cellular mobile radio networks. However, currently, these technologies do not yet have the desired average accuracies. Infrared systems and RFID systems are not generally available with complete coverage and are bound to specific requirements.
Due to the increasing distribution of wireless radio networks based, for example, on the WLAN standard, these wireless networks offer themselves as the basis for new localization methods.
Common localization methods previously used are, for example, based on triangulation, neighborhood relationships, lateration by means of time measurement or lateration by means of field strength evaluation. These methods are localization methods where either a position of stationary radio transmitters of base stations has to be known, or where training has to be previously performed at reference positions in an environment to be covered by the localization method.
In WLAN based locating systems, frequently, so-called received signal strength (RSS) fingerprinting is used as a basic method. This method is based on the assumption that signal strengths of radio signals of several radio stations received or receivable at a current location uniquely characterize the current location or the current position. If a reference database exists, which includes, for a number of reference locations or reference positions, transmitter identifications of radio stations received or receivable there at reference times, as well as the signal strengths of the corresponding radio signals, the current position can be inferred from a set of current measurement values (transmitter identification and associated signal strength values) by matching between currently measured measurement values and the reference values of the database. This matching evaluates for every reference point how similar its previously recorded measurement values or reference values are to the current measurement values of the current positions. The most similar reference point(s) is/are then used as a basis for an estimated value for the current location of the mobile terminal device.
The signal strength of a radio transmitter receivable at a reference position at a reference measurement time is determined experimentally for a reference database by a reference measurement. This results in a database including, for every reference position where a reference measurement has been performed, a list of radio transmitters (access points) including the respective associated received field strength and quality. This list can also be referred to as reference packet. With a WLAN implementation, such a reference database can, for example, include the following parameters:
RIDMACRSSIPGSXYZMAPNRCREATED100.0D.54.9E.17.8146530100579515627150012.03.0712:42100.0D.54.9E.1A.BA6726090579515627150012.03.0712:42100.0D.54.9E.1D.647200288579515627150012.03.0712:42100.0E.6A.D3.B9.8B59531100579515627150012.03.0712:42100.0F.A3.10.07.6C4646496579515627150012.03.0712:42100.0F.A3.10.07.FB7448894579515627150012.03.0712:42100.0F.A3.10.09.SF7237597579515627150012.03.0712:42200.0D.54.9E.17.81541381001439915451150012.03.0712:43200.0D.54.9E.18.1D76560111439915451150012.03.0712:43200.0D.54.9E.1A.BA62318941439915451150012.03.0712:43200.0D.54.9E.1D.6471348961439915451150012.03.0712:43200.0E.6A.D3.B9.8B453931001439915451150012.03.0712:43200.0F.A3.10.07.6C66853961439915451150012.03.0712:43200.0F.A3.10.07.FB722511001439915451150012.03.0712:43200.0F.A3.10.09.5F70990901439915451150012.03.0712:43300.0D.54.9E.17.81582911002458315627150012.03.0712:43300.0D.54.9E.18.1D78610682458315627150012.03.0712:43300.0D.54.9E.1A.BA62153982458315627150012.03.0712:43300.0D.54.9E.1D.6464187902458315627150012.03.0712:43300.0E.6A.D3.B9.8B328511002458315627150012.03.0712:43300.0F.A3.10.07.6C69006962458315627150012.03.0712:43300.0F.A3.10.07.FB71749922458315627150012.03.0712:43300.0F.A3.10.09.5F71482832458315627150012.03.0712:43300.0F.A3.10.09.8071000402458315627150012.03.0712:43The table includes the following information:reference position identification (RID)MAC addresses of the received stationsreceived field strengths of the radio transmitters RSSI (received signal strength indicator); 46560 means −46.560 dBm)reference position in Cartesian metric coordinates (x, y, z; 24583 means 245.83 m), as well astime of recording the measurement value.
The column PGS (“Percentage Seen”) indicates how often this station has been seen on a percentage basis when recording the measurement values (i.e. PGS=90 means that the station has been measured on average in 9 out of 10 measurements).
In the table presented above, all the information associated with a reference position identification (RID) correspond to a reference measurement packet. This means that the above exemplary table comprises three reference measurement packets corresponding to three different geographical reference positions.
For localization, currently received radio transmitters with their respective associated received field strength (measurement packet) are compared to reference packets from the reference database in a matching phase. Reference packets having a small distance to the current measurement packet, i.e. many common radio transmitters and few differing received field strengths match the current measurement packet value well. The reference positions belonging to the well-matching reference packets are very likely and are entered into a position calculation phase. An estimated value for the current position result, for example, from a reference position associated with a reference packet most similar to the current measurement packet, or from an interpolation of several reference positions associated with similar reference packets.
A conventional distance formula frequently used in the matching phase
                    acc        =                              ∑                          n              =              1                        Neq                    ⁢                      Δ            ⁢                                                  ⁢                          RSSI              n                                                          (        1        )            assumes that all radio transmitters can be received everywhere. Here, acc means the distance between current measurement packet and reference packet, and Neq a number of radio transmitters of which transmitter identifications previously recorded at the reference position are identical with transmitter identifications provided at the current position. Differences of RSSI values of radio transmitters of which transmitter identifications previously recorded at the reference position are identical to transmitter identifications provided at the current position, are referred to as ΔRSSIn (n=1, . . . , Neq). However, it is not at all costs the case that all radio transmitters are receivable everywhere. If a reference packet includes radio transmitters A, B and C, a current measurement packet the radio transmitters D, E, an (optimum) value of 0 results for the distance. Apparently, the reference packet matches perfectly, although no single radio transmitter between reference and current measurement packets matches.
Hence, in practice, calculation of the correspondence or distance can be modified, in that radio transmitters received in excess or too little in the current measurement packet compared to a reference measurement packet increase the distance by a fixed value resulting, for example, from a penalty function.
                    acc        =                                                                                                  EQW                    ·                                                                  ∑                                                  n                          =                          1                                                Neq                                            ⁢                                              Δ                        ⁢                                                                                                  ⁢                                                                              RSSI                            n                                                    ⁡                                                      (                                                                                                                  )                                                                                                                                +                                                            (                                              1                        -                        EQW                                            )                                        ·                                                                                                                        (                                                                                    ∑                                                  m                          =                          1                                                                          N                          nh                                                                    ⁢                                                                        M                                                      nh                            ,                            m                                                                          ⁡                                                  (                                                                                                          )                                                                                      +                                                                  ∑                                                  r                          =                          1                                                                          N                          HTM                                                                    ⁢                                                                        M                                                      htm                            ,                            r                                                                          ⁡                                                  (                                                                                                          )                                                                                                      )                                                                                        N              eq                        +                          N              nh                        +                          N              htm                                                          (        2        )            
Here, EQW means a weight between a 0 and 1 indicating how heavily the distance of measurement values or distance of signal strength values ΣΔRSSIn is to be evaluated compared to radio transmitters (Nnh) received in excess (Nhtm) or too little at the current position. For every radio transmitter missing in the reference values but included in the currently measured measurement values, a penalty value Mhtm,r( ) (m=1, . . . , Nhtm) can be defined. Also, for every radio transmitter included in the reference values but missing in the currently measured measurement values, a penalty value Mnh,m( ) (m=1, . . . , Nnh) can be defined.
The handling of different radio transmitters between a reference packet and a current measurement packet might heavily influence localization accuracy. A radio transmitter missing in the reference measurement packet but appearing in the current measurement packet has either been newly erected or is a strong indication that this fingerprint does not match.
During moving measurements or during continuous detection of measurement packets with mobile terminal devices, it can happen that, for example by noise or shading effects, a signal of a radio transmitter unreliably receivable at a current position is not measurable, at least in some sections. Here, a moving measurement means a measurement where the mobile terminal device moves continuously for measurement purposes and no stop is made at a certain position for measurement purposes (as in reference measurements). For the matching phase, this can have the effect that for an unreliably receivable radio transmitter a penalty value would erroneously be calculated for determining a measure of correspondence at a measurement time where the same is currently not receivable, although the radio transmitter had, for example, still been receivable shortly before that. The penalty value will result in a poorer measure of correspondence between current measurement packet and reference packet and, hence, to a poorer position estimation compared to a field strength consideration of the unreliably receivable radio transmitter at the current measurement time in the matching phase.